I am the defacto primary gunsmith for the MIT collegiate pistol team. They have a large number of Czech CO2 pistols that were made about 20 or more years ago. They shoot very accurately, and are generally pretty reliable. The primary firing seal is dying on a few of them, and the replacements we have don't fit due to a design change at some point.The seal is a short hard white plastic tube, press fit into a hole in the pistol frame. The actual firing seal is formed with a conical pin that presses into the hole through the center of the plastic seal. On the theory that the most likely seal material is Nylon 6/6, I bought some rod stock, and I'm going to try making new seals from scratch.
The recommended approach for removing the old seals it to run a brass wood screw into the seal & then pull it out. The pulling part takes quite a bit of force, and the process means the seal can't be re-used once removed. However, the one I extracted is sufficiently intact to take some measurements from. Give or take a few tenths, the seal measures 0.3157" OD. The bore in the pistol measures 0.3138" ID.
I've done some reading on both Nylon & press fitting plastics, and there are several issues that complicate matters:1) Nylon is hygroscopic, and will swell depending on the moisture content. That means if I machine one now when it is humid, it will shrink in the winter. On the otherhand, machining it will heat it a bit & dry it out slightly.
2) Nylon has a pretty high thermal coefficient of expansion. That means I probably need to let it cool off to measure it when I'm machining it. They recommend chilling the part to shrink it for the press fitting, but with high humidity, if I get it at all cool, it will be covered with moisture (if not frost). I'd have to drop the temperature ~ 60 degrees F to shrink the part by 1 mil, which definitely gets it into the frost category.3) If I make it sufficiently oversized to make sure that moisture & thermal variations don't allow it to shrink enough to be a problem, it can creep over time & loosen up. I suspect that may be why some of them are failing after all this time, possibly shrinking enough to allow CO2 to sneak past them. There's a good web site that talks about this here: This also means that the measured size of the seal I removed is probably smaller than it was originally cut. As extracted, they had about a 2 mil interference fit, which would seem to be plenty. I need to double check the calculations, but that should result in over 1500 psi of expansive force between the seal & the bore of the hole. It's also enough to produce significant creep over time: 1% is over 3 mils, although as it creeps, the forces will drop, and presumably it will reach something like steady state when the force drops enough.
4) Annealing: Lots of folks recommend annealing Nylon before trying to machine it. From some DuPont literature, 3/8" rod stock should only require an hour at a couple hundred F. They recommend not doing it in air, and suggest heating it in wax or oil, and then cooling it slowly & evenly to avoid surface stresses. I'm thinking heating it in cooking oil with a thermometer, and then letting the whole mess cool slowly.
There are other plastics that have lower moisture issues than Nylon 6/6, but I think the TCE & creep is going to be similar for a lot of them.
I seem to have opened a serious can of worms here. I'm thinking about just going for it, cutting one 2.5 mils oversized without annealing. If anyone has any real experience to help sort through the chaos, I'm all ears.